1. WoW cost $100 million for development and total upkeep for 4 years (Source)
2. WoW cost $200 million for development and total upkeep for 4 years (Source)
3. WoW cost $200 million for total upkeep but not development (Source)
4. WoW cost $100 million for development and/or total upkeep (Source)
5. WoW cost $63 million for development that took 4.5 years (Source)

If you read the articles and comments, you can certainly understand that there is a lot of confusion over this number.

Let's take the most expensive number - that WoW upkeep costs $200 million over 4 years (it's 47 months actually). On average, that's $4.25M per month.

That number seems extremely high considering that there are only ~2.5k servers (~15k blades / ~730 realms worldwide) utilized. Until that is, you read this:

"That's $200 million for the total cost of upkeep since the game's November 2004 release (presumably not including the initial cost to develop the game). This includes payroll for the entire staff, hardware support, and -- apparently the biggest infrastructure cost -- customer service."

That makes more sense: the biggest upkeep cost in that $200 million figure is customer service. So, that $4.25M per month figure is probably a lot less now considering that there have been significant numbers of lay-offs in the WoW offices and support teams, and that the server count has pretty much remained the same since the announcement.

Also, some Blizzard employees have stated that they have reduced yearly operating costs by $500,000 and have introduced new policies that have resulted in multi-millions of yearly loss reduction. (Link)

So it's safe to say that all World of Warcraft operating costs are at max $4.25 million per month. Realistically though, it's probably half of that amount or less now.

Keep this figure in mind, it's important for another post that I've been preparing. :)